1. Field of the Invention
The present invention relates to stencil printer and more particularly to a stencil printer capable of selectively operable in a simplex print mode using a simplex master or a duplex pint mode using duplex master.
2. Description of the Background Art
Digital thermosensitive stencil printing, which is well known in the art as a simple printing method, uses a stencil printer including a thermal head having an array of heating elements arranged thereon. While the thermal head is in contact with a stencil being conveyed, current is selectively fed to the heating elements in the form of pulses for thereby selectively perforating, or cutting, the stencil with in accordance with image data. After the perforated stencil, or master as referred to hereinafter, has been wrapped around a porous, cylindrical print drum, a press roller or similar pressing means presses the print drum via a sheet. Consequently, ink is transferred to the sheet via the perforations of the master and form an image on the sheet.
A current trend in the stencil printers art is toward duplex prints carrying images on both surfaces thereof and therefore saving, e.g., sheets and a space necessary for the storage of documents. It has been customary to produce a duplex copy by feeding a sheet from a sheet feeding section to a printing section to thereby print an image on one surface of the sheet, reversing and again feeding the sheet to the printing section to thereby print an image on the other side of the same sheet. Such a procedure, however, forces the operator to again set sheets once discharged and neatly arrange sheets each carrying an image on one surface thereof by hand. Another problem with the conventional duplex printing is that it needs two times longer net period of time than simplex printing because a single sheet must be passed via the printing section two times.
In light of the above, Japanese Patent Laid-Open Publication No. 2003-200645 discloses a duplex printer using a duplex print formed with a first and a second image arranged in the direction of rotation of a print drum. In the duplex printer taught in this document, after one of the first and second images has been formed on a first sheet fed from a sheet feeder, the first sheet is guided to an auxiliary tray. After one of the images of the duplex master has been formed on a second sheet also fed from the sheet feeder, the second sheet is also guided to the auxiliary tray. At the same time, the first sheet is again fed from the auxiliary sheet to the printing section, so that the other image is formed on the reverse surface of the sheet. The first sheet is then discharged as a duplex copy. Such a procedure is repeated to produce duplex prints by a single step.
However, the duplex printer taught in the above document prints an image on one surface of a sheet and then prints an image on the other surface of the same sheet. This brings about a problem that when an image is to be printed on the other surface of a sheet, which carries an image on one surface, ink is transferred from the one surface to pressing means and then transferred to one surface of the next sheet (so-called offset).
To cope with offset, pressing means my be provided with a fine, irregular surface configured to allow ink to deposit on the surface little. However, the pressing means contacts a master wrapped around the print drum when a smaller sheet size is selected after a master has been made, when, after a master for duplex printing has been made, simplex printing is effected with sheets smaller in size than the master or when, after master making, the print drum is replaced with a new print drum loaded with a simplex or a duplex master greater in size than sheets to be used. In any case, ink fed from the inside of the print drum is directly transferred to the surface of the pressing means. It is extremely difficult to remove ink filled gaps between fine projections present on the surface of the pressing means.